gpg: Fix _ostree_gpg_verifier_add_keyring()

The function never fails, but its API makes it look like it can.

Fortunately it's private, so just fix it.

src/libostree/ostree-gpg-verifier.c

@@ -259,16 +259,13 @@ out:
   return result;
 }
 
-gboolean
+void
 _ostree_gpg_verifier_add_keyring (OstreeGpgVerifier  *self,
-                                  GFile              *path,
-                                  GCancellable       *cancellable,
-                                  GError            **error)
+                                  GFile              *path)
 {
-  g_return_val_if_fail (path != NULL, FALSE);
+  g_return_if_fail (G_IS_FILE (path));
 
   self->keyrings = g_list_append (self->keyrings, g_object_ref (path));
-  return TRUE;
 }
 
 gboolean

src/libostree/ostree-gpg-verifier.h

@@ -51,8 +51,7 @@ gboolean      _ostree_gpg_verifier_add_keyring_dir (OstreeGpgVerifier   *self,
                                                     GCancellable        *cancellable,
                                                     GError             **error);
 
-gboolean      _ostree_gpg_verifier_add_keyring (OstreeGpgVerifier   *self,
-                                                GFile               *path,
-                                                GCancellable        *cancellable,
-                                                GError             **error);
+void _ostree_gpg_verifier_add_keyring (OstreeGpgVerifier *self,
+                                       GFile             *path);
+
 G_END_DECLS

src/libostree/ostree-repo.c

@@ -3376,9 +3376,7 @@ _ostree_repo_gpg_verify_with_metadata (OstreeRepo          *self,
     }
   if (extra_keyring != NULL)
     {
-      if (!_ostree_gpg_verifier_add_keyring (verifier, extra_keyring,
-                                             cancellable, error))
-        goto out;
+      _ostree_gpg_verifier_add_keyring (verifier, extra_keyring);
     }
 
   if (metadata)